The present invention generally relates to a dielectric filter which has band elimination characteristics with a resonance circuit having a dielectric resonator as a main body being connected through a coupling inductance.
FIG. 12 shows a conventional dielectric filter having band elimination characteristics. This filter is provided with a plurality of resonance circuits with capacitors 15 used for stray capacitance connected in parallel with a series circuit between a dielectric resonator 14 and a capacitor 13 used for external coupling.
In such a filter assembly construction as described above, a plurality of resonators 14 are accommodated within case 20, with internal conductors 14a being horizontal. A bent portion 17a on the proximal end of a ground plate 17 with a distal portion being grasped between the internal bottom face of case 20 and an external conductor 14b on the outer periphery of the resonator 14 is opposed in the horizontal direction by a coupling terminal 16 which is soldered to the internal conductor 14a and is externally exposed. The capacitors 13 and 15 are mounted one after another with solder 18 or the like between the opposing coupling terminal 16 and the bent portion 17a. The resonator 14 is supported by elastic pressure exerted by a spring 19 engaged between the resonator 14 and the upper internal face of the case 20, which opposes pressure exerted by the distal end of the ground plate 17 which is shaped into a wavelike form and is engaged between the bottom internal face of the case 20 and the resonator 14. Reference numeral 12 in FIG. 12 is a transmission line including a coupling inductance 11 engaged between the capacitor 13 and the capacitor 15, showing the cross of the transmission line.
In the above construction, separate capacitors 13 and 15 are used. In order to obtain the desired filter characteristics, the inductance and capacitance of each stage must be specified at a given value. Since all the values thereof are generally different, the parts which have various capacitances are conventionally required to be controlled for each determined value of capacitance.
Also, in the above described construction, since it is required to mount the capacitor to the resonator without any additional support, the assembling operation cannot be easily effected, thus resulting in improper mass production. The mounting operation is unstable, even when the soldering operation and other steps are effected with the capacitor being supported by grasping pressure between the coupling terminal 16 and the bending portion 17a by the use of the elasticity of the ground plate 17.
Also, as the space between the mounting portion A of the capacitor and the case connecting portion B in the ground plate 17 becomes longer, surplus inductance is caused resulting in unstable electric characteristics.
Further, since the capacitor is unsupported, the mechanical strength is weaker, so that the solder joint or the like may come off because of vibrations and the like.